Neutral base recombination and its influence on the temperature dependence of Early voltage and current gain-Early voltage product in UHV/CVD SiGe heterojunction bipolar transistors

We present the first comprehensive investigation of neutral base recombination (NBR) in ultra-high vacuum/chemical vapor deposited (UHV/CVD) SiGe heterojunction bipolar transistors (HBT's), and its influence on the temperature characteristics of Early voltage (V/sub A/) and current gain-Early voltage product (/spl beta/V/sub A/). We show that a direct consequence of NBR in SiGe HBT's is the degradation of V/sub A/ when transistors are operated with constant-current input (forced-I/sub B/) as opposed to a constant-voltage input (forced-V/sub BE/). In addition, experimental and theoretical evidence indicates that with cooling, V/sub A/ in SiGe HBT's degrades faster than in Si bipolar junction transistors (BJT's) for forced-I/sub B/ mode of operation. Under the forced-V/sub BE/ mode of operation, however, SiGe HBT's exhibit a thermally-activated behavior for both V/sub A/ and /spl beta/V/sub A/, in agreement with the first-order theory. The differences in V/sub A/ as a function of the input bias and temperature for SiGe HBT's are accurately modeled using a modified version of SPICE. The performance of various practical SiGe HBT circuits as a function of temperature, in the presence of NBR, is analyzed using this calibrated SPICE model.

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